The present invention relates to a semiconductor device equipped with an oscillator circuit, and a control method thereof.
A clock signal used in a semiconductor device is generated typically by using a crystal resonator and an oscillator circuit. It is desirable that noise immunity of the oscillator circuit is high to obtain a stable clock signal.
A Patent Document 1 has disclosed an oscillator circuit. The oscillator circuit has an inverter having a transistor, a feedback resistor, capacitive elements, a variable current source, a timer circuit, and a current controller. The feedback resistor is coupled in parallel with the inverter. The capacitive elements are respectively provided on the input and output sides of the inverter. The variable current source supplies either of two types of currents different in level to the inverter. The timer circuit counts a prescribed time from the start-up of a power supply for the oscillator circuit. The current controller controls the variable current source in such a manner that it supplies the current large in level, of the two types of currents suppliable by the variable current source to the inverter during a period taken until the count time by the timer circuit goes beyond a predetermined time and that after it has gone beyond the predetermined time, it supplies the current small in level to the inverter.
A Patent Document 2 has disclosed a crystal oscillator circuit in which a power supply line is made identical with other electronic circuits. A low-pass filter for preventing noise superimposed on the power supply line from entering the crystal oscillator circuit is provided between the power supply line and the crystal oscillator circuit.
A Patent Document 3 has disclosed a crystal oscillator. The crystal oscillator is equipped with a crystal oscillator circuit and a buffer circuit. The crystal oscillator circuit is coupled to a power supply circuit and takes the output of a control voltage generating circuit as an input. The buffer circuit is coupled to the power supply circuit and takes the output of the crystal oscillator circuit as an input. Further, a low-pass filter is coupled between the power supply circuit and the buffer circuit.